Any word on custom closures, or new ones?
This is, in my eyes, not even theoretically possible to implement in Cycles. The backside of an object cannot be influenced by a light source on the front, simply because Cycles works with samples from the camera over the surface towards the lamp. There is no way that a sample bouncing off the surface on the back could directly end up in the light on the other side as there is an object in-between.

However, I don't think this shader is necessary in Cycles. It was built to allow even light setups with very few lights to not look over-contrasted and harsh. With real global illumination, which you have in Cycles, light bouncing off of other objects around your object will automatically (at least with more than 0 diffuse-bounces) provide a smooth light distribution around your object from a single light source, which is exactly the effect the Half-Life developers tried to fake.